Analog delay lines and adaptive biasing

ABSTRACT

Examples of analog delay lines and analog delay systems, such as DLLs incorporating analog delay lines are described, as are circuits and methods for adaptive biasing. Embodiments of adaptive biasing are described and may generate a bias signal for an analog delay line during start-up. The bias signal may be based in part on the frequency of operation of the analog delay line.

TECHNICAL FIELD

Embodiments of the invention relate generally to semiconductor devicesand, some examples particularly to analog delay lines.

BACKGROUND

In many electronic devices, for example memory devices, components maybe clocked by an external clock signal and may perform operations atpredetermined times relative to the rising and falling edges of theapplied clock signal. Examples of synchronous integrated circuitsinclude synchronous memory devices such as synchronous dynamic randomaccess memories (“SDRAMs”), synchronous static random access memories(“SSRAMs”), and packetized memories like SLDRAMs and RDRAMs, and includeother types of integrated circuits as well, such as microprocessors. Thetiming of signals external to a synchronous integrated circuit may bedetermined by an external clock signal, and operations within thesynchronous integrated circuit may be synchronized to externaloperations. For example, commands may be placed on a command bus of amemory device in synchronism with an external clock signal, and thememory device may latch these commands at the proper times tosuccessfully capture the commands. To latch the applied commands, aninternal clock signal may be developed in response to the external clocksignal, and is typically applied to latches contained in the memorydevice to clock the commands into the latches. The internal clock signaland external clock should be synchronized to ensure the internal clocksignal clocks the latches at the proper times to successfully capturethe commands. In the present description, “external” may refer tosignals and operations outside of the memory device, and “internal” mayrefer to signals and operations within the memory device. Moreover,although the present description includes description of synchronousmemory devices, the principles described herein are equally applicableto other types of synchronous integrated circuits or to thesynchronization of generally any periodic signals.

To synchronize external and internal clock signals in modern synchronousmemory devices, a number of different approaches have been consideredand utilized, including delay locked loops (“DLLs”), as will beappreciated by those skilled in the art. Generally, these approachesutilize a delay line containing one or more delay elements to delay aninput periodic signal and feed back a phase difference-related signalbetween the input and the output to control the amount of delay providedby the delay line. In this matter the output periodic signal may be“locked” to the input periodic signal.

FIG. 1 is a schematic illustration of a general delay locked loop 100.An input signal 105, Sig_in, may be provided to a buffer 107 which maygenerate the buffered signal Sig_buff 109. The buffered signal may beprovided to a delay line 110 that is configured to delay the bufferedsignal in accordance with a control signal sig_cntrl 112 to generate anoutput signal, sig_out 114. To generate the control signal 112, a phasedetector 120 and control logic 125 may be provided. The phase detector120 may compare a phase of the output signal sig_out 114 with that ofthe buffered input signal sig_buff 109. The phase detector 120 maygenerate a signal corresponding to a phase difference between sig_buff109 and sig_out 114. The signal corresponding to the phase differencemay be provided to the control logic 125, which may include, forexample, a charge pump and/or a loop filter, which may in turn generatethe control signal sig_contrl 112. Although not shown in FIG. 1,additional components such as multiple delay lines (for example, acoarse and fine delay line), or mock delays in the feedback path mayalso be included.

As speeds of electronic devices continue to increase, timingrequirements for DLLs such as those shown in FIG. 1 are increasing.Jitter and/or skew may be caused by variations in the power supplyvoltage(s) which power the elements in the delay line 110. Voltageregulators may be used in an effort to maintain a constant power supplyvoltage and reduce or eliminate jitter or skew caused by power supplyvoltage variations.

Power supply voltages, however, are also decreasing in many electronicdevices. It may be infeasible to continue to use voltage regulators tomaintain constant power supply voltages at lower voltages.

Generally, the delay line 110, phase detector 120, and control logic 125may be implemented using digital circuitry. For example, the delay line110 may include multiple individual delay elements (e.g. stages) coupledin series, with each individual delay element delaying the input signalan amount and providing the delayed input signal to the next element,until the output signal sig_out 114 is generated. Each delay element istypically implemented using two NAND gates. The control logic 125 istypically configured to provide a digital signal to the delay line 110.That is, the sig_cntrl 112 is typically a digital signal.

Analog delay elements have been considered for use in the delay line110, however, analog delay elements typically consume a larger area andmore power than their digital counterparts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a general delay locked loop.

FIG. 2 is a schematic illustration of a DLL including an analog delayline in accordance with an embodiment of the present invention.

FIG. 3 is a schematic illustration of an analog delay line in accordancewith an embodiment of the present invention.

FIG. 4 is a schematic illustration of an analog delay element arrangedin accordance with an embodiment of the present invention.

FIG. 5 is a schematic illustration of bias control circuitry arranged inaccordance with an embodiment of the present invention.

FIG. 6 is a schematic illustration of bias control circuitry arranged inaccordance with another embodiment of the present invention.

FIG. 7 is a schematic illustration of a DLL including an exampleimplementation of the frequency measurement circuitry according to anembodiment of the present invention.

FIG. 8 is a schematic illustration of a DLL incorporating an analogdelay line according to an embodiment of the present invention.

FIG. 9 is a schematic illustration of a portion of a memory according toan embodiment of the present invention.

DETAILED DESCRIPTION

Certain details are set forth below to provide a sufficientunderstanding of embodiments of the invention. However, it will be clearto one skilled in the art that embodiments of the invention may bepracticed without various of these particular details. In someinstances, well-known circuits, control signals, timing protocols,and/or software operations have not been shown in detail in order toavoid unnecessarily obscuring the described embodiments of theinvention. For example, embodiments described below include descriptionof differential circuits and systems. It is to be understood thatsingle-ended circuits and systems may be used in other embodiments.

As described generally above, variations in power supply voltage(s) mayintroduce undesirable jitter and/or skew into the operation of DLLs.Analog delay elements may exhibit improved power supply sensitivity,however, they have conventionally been considered to consume anunacceptable amount of power and/or area. Embodiments of the presentinvention provide delay lines having analog delay elements. Delayelements and systems are described which may reduce the amount of areaand/or power consumed by a DLL using embodiments of the described delaylines. Further, embodiments of adaptive biasing techniques and circuitryare described which may advantageously improve the lock range of ananalog delay line, and/or the area required for an analog delay line.The description of advantages provided herein is to aid in understandingembodiments of the invention, and it is to be understood that not allembodiments may provide all described advantages, and some embodimentsmay not provide any of the described advantages.

FIG. 2 is a schematic illustration of a DLL 200 including an analogdelay line in accordance with an embodiment of the present invention.The DLL 200 may include an analog delay line 205. A buffer 202 may becoupled to the analog delay line 205. A phase detector 207 may becoupled to an input and an output of the analog delay line 205. Controllogic 209 may be coupled to the Phase Detector 207 and the analog delayline 205. Further, frequency measurement circuitry 210 may be coupled tothe analog delay line 205. Bias control circuitry 215 may be coupled tothe frequency measurement circuitry 210 and the analog delay line 205.

The analog delay line 205 may include a plurality of analog delayelements coupled in series, examples of which will be described furtherbelow. Portions of the analog delay elements may share bias circuits, aswill be described further below. Sharing bias circuits mayadvantageously reduce area and/or power required to implement the analogdelay line 205 in some embodiments. Analog delay elements in the analogdelay line 205 may be adaptively biased, as will be described furtherbelow. Adaptive biasing may also advantageously improve locking range,reduce area and/or reduce power required to implement the analog delayline 205.

The phase detector 207 and control logic 209 may generally beimplemented using any circuitry suitable for adjusting a delay of theanalog delay line 205 based on a comparison of the input and outputsignals. In embodiments of the present invention, the phase detector 207and control logic 209 may be implemented using digital circuitry ofconventional DLLs. That is, in some embodiments, the analog delay line205, while using analog delay elements instead of digital delayelements, may nonetheless be configured to receive a conventionaldigital control signal, as will be described further below.

The frequency measurement circuitry 210, as will be described furtherbelow, is configured to measure a frequency of operation of the analogdelay line 205, for example, the frequency of the buffered signalsig_buff or one of the input signals to one of the intermediate delayelements in the delay line. The frequency measurement circuitry 210 mayprovide an indication of the frequency of operation to the bias controlcircuitry 215 (e.g. a control signal). Examples will be describedfurther below, however, substantially any circuitry suitable formeasuring a frequency of operation of the analog delay line 205 may beused.

The bias control circuitry 215, as will be described further below, isconfigured to generate a bias voltage and/or current for the analogdelay line 205. The bias control circuitry 215 may receive theindication of frequency of operation from the frequency measurementcircuitry 210, and in this manner, the bias voltage and/or current maybe based on the frequency of operation.

During operation, an input signal 201, sig_in, may be provided to thebuffer 202. Generally, any periodic signal may be provided as the inputsignal 201, including a clock signal. The buffer may generate a bufferedinput signal sig_buff. The analog delay line 205 may delay the bufferedinput signal through a series of analog delay elements to generate anoutput signal sig_out. The phase detector 207 may receive the outputsignal sig_out and the buffered input signal sig_buff. The phasedetector 207 may generate a signal indicative of a phase differencebetween the output signal and the buffered input signal, and provide thesignal to the control logic 209. The control logic 209 may generate acontrol signal setting a delay time for one or more elements of theanalog delay line 205 and/or selecting a number of delay elements foruse in the analog delay line 205. That is, the control logic 209 mayspecify an output location from the analog delay line 205, selecting anumber of delay elements through which the buffered input signal ispropagated to generate the output signal.

A bias voltage and/or current may be provided to the analog delay line205 by the bias control circuitry 215. The frequency measurementcircuitry may be coupled to the analog delay line 205 to measure afrequency of the periodic signal passed through the analog delay line205. The frequency measurement circuitry 205 may provide a signal to thebias control circuitry 215 indicative of the frequency of operation ofthe analog delay line 205. Based on the frequency of operation, and insome examples, with a comparison of a control voltage or current with areference, the bias control circuitry 215 may generate a bias signal forthe analog delay line 205, which may in turn be used to generate a biasvoltage or current used by one or more of the delay elements. The biasvoltage and/or current may be established in this manner during astartup mode of operation of the DLL 200. Accordingly, the bias voltageand/or current may be established once, and the DLL operated using theestablished voltage and/or current. In other embodiments, the biasvoltage and/or current may be periodically updated.

FIG. 3 is a schematic illustration of analog delay line 205 inaccordance with an embodiment of the present invention. The analog delayline 205 includes analog delay elements 301-308 coupled in series. Aswill be described further below, each of the analog delay elements301-308 includes an ExitEn input to receive an ExitEN signal, which maybe provided by the control logic 209 of FIG. 2. The ExitEn signal mayspecify which of the delay elements the output signal is taken from.Each of the analog delay elements 301-309 further includes a dlyEN inputto receive a dlyEn signal, which may also be provided by the controllogic 209 of FIG. 2. The dlyEn signal may specify which of the analogdelay elements 301-308 should be enabled to delay an input signal.

Each of the analog delay elements 301-309 further includes an ina andinb input, configured to receive a differential input signal, such as adifferential version of the signal sig_buff of FIG. 2. Each of theanalog delay elements 301-309 further includes an nbias and pbias inputconfigured to receive bias signals for NMOS and PMOS transistors,respectively. The nbias and pbias signals may be provided by the biascontrol circuitry 215 of FIG. 2 in some embodiments.

Each of the analog delay elements 301-309 includes the outa and outboutputs for providing a delayed version of the ina and inb signals tothe ina and inb inputs of the next delay element. Each of the analogdelay elements 301-309 also includes a cka and ckb output for providingthe delayed version of the respective ina and inb inputs to an outputline of the analog delay line 205—the lines ckOa and ckOb, or ckEa andckEb, based on (e.g. responsive to) the ExitEN signal. The analog delayline 205 of FIG. 3 is of a type where both an even and an odd outputsignal may be provided, which may later be mixed to improve a resolutionof the delay line. Accordingly, the delay elements 301, 303, 305, and307 have their outputs cka and ckb coupled to the odd output line ckOaand ckOb, respectively. The delay elements 302, 304, 306, and 308 havetheir outputs cka and ckb coupled to the even output line ckEa and ckEb,respectively. In this manner, an odd and even signal may be output fromthe analog delay line that are separated by a single unit delay amount.These signals may be mixed, as known in the art, to achieve improvedresolution of a DLL incorporating the delay line 205.

Some components of the analog delay elements 301-308 may share loadand/or bias circuits. The sharing of load and/or bias circuitry mayreduce the area required for the delay line 205. For example, the odddelay elements 301, 303, 305, and 307, may share a bias circuit 310. Thebias circuit 310 is implemented as an NMOS transistor that receives thenbias signal at its gate terminal. The bias circuit 310 will accordinglyprovide a current based on the nbias signal and transistorcharacteristics of the transistor. In an analogous manner, the evendelay elements 302, 304, 305, and 308 may share a bias circuit 312,again implemented as an NMOS transistor that receives the nbias signalat its gate terminal.

The cka outputs of the odd delay elements 301, 303, 305, and 307, may becoupled to the load circuit 314. The load circuit 314 may be implementedas a pair of PMOS transistors, with one configured to receive the pbiassignal at its gate terminal, and the other having its gate terminal tiedto its drain, which is tied to the drain of the first transistor, asshown. The load circuit 314 may accordingly provide a load to the analogdelay elements based on the pbias signal and transistor characteristics.In a similar manner, the ckb outputs of the odd delay elements 301, 303,305, and 307 are coupled to the load circuit 316, again implemented as apair of pmos transistors. The cka outputs of the even delay elements302, 304, 306, and 308, are coupled to the load circuit 318. The ckboutputs of the even delay elements 302, 304, 306, and 308, are coupledto the load circuit 320. Sharing load and/or bias circuits among thedelay elements may reduce an area required to layout the analog delayline 205. In other embodiments, load and/or bias circuits may not beshared.

FIG. 4 is a schematic illustration of an analog delay element 301arranged in accordance with an embodiment of the present invention. Theanalog delay element 301 includes a delay amplifier 405 and amultiplexer 410. The delay amplifier 405 is configured to delay adifferential input signal ina and inb to generate the delayed outputsignal outa and outb, responsive to the dlyEn signal. The multiplexer410 is configured to couple the outa and outb signals to the cka and ckboutputs responsive to the ExitEn signal. Recall that the cka and ckboutputs may be provided to the even and odd output lines, ckOa, ckOb,and ckEa, ckEb.

The delay amplifier 405 includes loads 420 and 421, one load for eachleg of the differential delay amplifier 405. The loads 420 and 421 areimplemented as a pair of pmos transistors, with one transistor of thepair receiving the pbias signal at its gate, and the other transistorhaving its gate coupled to its drain. The loads 420 and 421 are coupledto differential amplifier transistors 430 and 431, respectively. Thetransistors 430 and 431 receive the ina and inb signals at their gates,respectively, and generate the outa and outb signals at their drains,respectively. Both differential amplifier transistors 430 and 431 arecoupled to the transistor 435. The transistor 435 is configured toreceive the dlyEn signal at its gate, and therefore to turn onresponsive to the dlyEn signal. Accordingly, the amplifier 405 isenabled responsive to the dlyEn signal turning the transistor 435 on.The bias transistor 437 is coupled to the enable transistor 435 and isconfigured to receive the nbias signal at its gate. The bias transistor437 accordingly provides a bias current to the amplifier 405 based onthe nbias signal and the characteristics of the transistor 437. Thedelay provided by the amplifier 405 will in part be based on (e.g.determined by) the bias current, and therefore the nbias voltage. Thedelay provided by the amplifier 405 may also be in part determined bythe load 420, and therefore by the pbias voltage. As will be describedfurther below, the nbias and pbias signals may be adaptively determinedto account for process variations and frequency of operation of thedelay line. Although the amplifier 405 is implemented using amplifiertransistors 430 and 431, substantially any analog delay elementconfiguration may be used.

The multiplexer 410 is configured to provide the outa and outb signalsto the cka and ckb outputs responsive to the ExitEn signal. By enablingthe multiplexer at a particular delay element, the number of delayelements used in the analog delay line 205 of FIG. 2 may be selected.The multiplexer 410 includes two multiplexer transistors 440 and 441configured to receive the outa and outb signals at their respective gateterminals and output cka and ckb signals at their respective drainterminals. The transistors 440 and 441 may be loaded by shared loads 314and 316 of FIG. 3. For example, as shown in FIG. 3, the load 314 may becoupled to the cka output, while the load 316 may be coupled to the ckboutput. Note that the cka and ckb outputs are also illustrated in FIG. 4and connect to the transistors 441 and 440, respectively. Thetransistors 440 and 441 are coupled to an enable transistor 445configured to receive the ExitEn signal at its gate. Accordingly theenable transistor 445 turns on responsive to the ExitEn signal andenables the cka and ckb signals to be output from the multiplexer 410.The multiplexer 410 may be biased at the CS input by shared bias circuit310 of FIG. 3. Note that the bias circuit 310 is shown in FIG. 3 coupledto the CS node of the element 301. In FIG. 4, the CS node is showncoupled to the transistor 445.

Each of the delay elements 301-308 of FIG. 3 may include an amplifierand multiplexer analogous to those shown in FIG. 4. The delay amplifiersand multiplexers may share load and bias circuitry as described withreference to FIGS. 3 and 4. Also note that each delay element 301-308may include only a single delay amplifier, such as the amplifier 405 ofFIG. 4. Although in other embodiments, more amplifiers may be used ineach delay element, in some embodiments only one amplifier is present.This may be in contrast to previous digital designs, where at least twoNAND gates were used in each delay element. Utilizing a single amplifierand multiplexer per delay element may reduce the area and power requiredto implement an analog delay line in accordance with embodiments of thepresent invention.

Accordingly, embodiments of analog delay lines and analog delay elementsthat may be used, for example, in DLLs for electronic devices, have beendescribed above. Analog delay lines incorporating analog delay elements,as opposed to digital delay elements, may have improved power supplysensitivity (PSS) and may advantageously not require power supplyregulation, even as power supply voltages decrease in electronicdevices. Embodiments of analog delay lines and elements described abovemay utilize a single amplifier per element, and may share load and/orbias circuitry, which may advantageously reduce the power supplyconsumption and area required to implement analog delay lines inaccordance with the present invention.

One drawback of analog delay lines may be a reduced lock range of a DLLincorporating the analog delay line. Lock range may be reduced relativeto digital delay lines, in part due to the biasing scheme used in analogdelay lines. Embodiments of adaptive biasing are further described belowwhich may advantageously generate a bias voltage and/or current for ananalog delay line based on the as-made process characteristics of theanalog delay line and/or the frequency of operation of the analog delayline. By adaptively generating a bias voltage and/or current, lock rangeof a DLL incorporating an analog delay line may be improved.

FIG. 5 is a schematic illustration of bias control circuitry arranged inaccordance with an embodiment of the present invention. The bias controlcircuitry 500 includes a control voltage generator 502 coupled to ahalf-replica bias circuit 505 coupled to a buffer element 510 which mayoutput the nbias and pbias signals to a delay element 515. The delayelement 515 is illustrated in FIG. 5 as a simplified delay element. Oneor more of the delay elements 301-308 of FIG. 3 may receive the nbiasand pbias signals as described above. The loads of the delay element 515are shown as variable resistors 516 and 517. However, as described withreference to FIGS. 3 and 4, transistors may also be used.

The bias control circuitry 500 is configured to generate the nbias andpbias signals based on a control voltage. The bias control circuitry 500includes a control voltage generator 502. The control voltage generatoris configured to generate a control voltage V_(ctrl). The controlvoltage is generated by drawing a current from an adjustable currentsource 513 through transistors 514 and 516. The amount of current 513may be adjustable and may be based on a comparison with a bandgapreference 501 and a frequency of operation of the analog delay line,which may be provided by the frequency measurement circuitry 210, as hasbeen mentioned above and will be described further below. The adjustablecurrent source 513 may be implemented using one or more current mirrors.The adjustable current source 513 may receive a reference signal fromthe bandgap reference 501 and a signal indicative of a frequency ofoperation from the frequency measurement circuitry 210. The adjustablecurrent source 513 may provide a current based on the bandgap referenceand the frequency of operation. In this manner, as the frequency ofoperation increases, the adjustable current source 513 may be adjustedto lower the control voltage V_(ctrl), and vice versa in some examples.The control voltage is generated at the common drains of the transistors514 and 516, and will be based on the value of the current provided bythe adjustable current source 513 and the supply voltage applied to thetransistors 514 and 516. The control voltage V_(ctrl) may then beapplied to a half-replica bias circuit 505.

The half-replica bias circuit 505 is configured to generate the nbiassignal based on a comparison between the control voltage V_(ctrl) and afeedback voltage. The half-replica bias circuit 505 includes an op-amp520 configured to receive the control voltage V_(ctrl) and a feedbackvoltage from a gate/drain connection of a pmos transistor 522. Thetransistor 521 is configured to receive V_(ctrl) at its gate terminal.The transistor 522 has its drain tied to the drain of the transistor 521and the gate of the transistor 522. The gate/drain of the transistor 522is also taken as the feedback voltage to the op-amp 520. The op-ampgenerates the nbias signal based on a comparison of the control andfeedback voltages. The nbias signal is also applied to the bufferelement 510, as shown. The pbias signal is generated at the gate of thepmos transistor 532.

In this manner, the nbias and pbias signals may be generated based on avariable control voltage. Accordingly, the nbias and pbias signals maybe adaptively generated (e.g. determined). The bias control circuitrymay be used as the bias control circuitry 215 of FIG. 2. Other biascontrol circuitry may be used in other examples for adaptivelygenerating bias signals for the analog delay line 205 of FIG. 2.

FIG. 6 is a schematic illustration of bias control circuitry arranged inaccordance with another embodiment of the present invention. A referencevoltage, V_(ref), may be generated by a resistor divider including theseries combination of resistors 650 and 652. The reference voltageV_(ref) may be generated at a node between the resistors 650 and 652,and may accordingly be set by the values of the resistors 650, 652, anda voltage applied across the resistors 650 and 652. In the example ofFIG. 6, the resistor 650 is coupled between a power supply voltage 654and V_(ref). The resistor 652 is coupled between V_(ref) and a node of atransistor 656. The transistor 656 is configured to receive an enablesignal, En, at its gate. Responsive to the enable signal, the transistor656 may turn on, allowing a current path between V_(ref) and a powersupply voltage 658 (ground in FIG. 6). Accordingly, V_(ref) may begenerated responsive to the enable signal En.

The voltage V_(ref) may be provided to an input terminal of anoperational amplifier 660. The operational amplifier is configured togenerate a bias voltage, nbias, based on the reference voltage V_(ref)and a control signal V_(ctrl) applied to another input terminal of theoperational amplifier 660. The control signal V_(ctrl) may be generatedbased on a value of a variable resistor 662. The value of the variableresistor 662 may be selected based on a signal received from thefrequency measurement circuitry 210. For example, the frequencymeasurement circuitry 210 may provide a signal indicative of a frequencyof operation of an analog delay line to the variable resistor 662. Theresistance of the variable resistor 662 may be set (e.g. adjusted) basedon the signal indicative of the frequency of operation. The variableresistor 662 is coupled between V_(ctrl) and the power supply voltage654. A transistor 664 may be coupled between V_(ctrl) and a transistor668. The transistor 664 may have its gate coupled to the power supplyvoltage 654, causing the transistor 664 to be turned ‘on’ andconductive. The transistor 664 may be coupled between V_(ctrl) and atransistor 668. The transistor 668 has a gate terminal coupled to anoutput terminal of the operational amplifier 660 and a terminal coupledto the power supply voltage 658. Accordingly, the transistor 668 mayreceive the bias voltage nbias at its gate terminal. Responsive to thenbias signal, the transistor 668 may become conductive, and the controlvoltage V_(ctrl) may accordingly be generated responsive to receipt ofthe nbias voltage at the gate of the transistor 668.

Generation of an nbias signal is shown in FIG. 6. The circuitry shown inFIG. 6 may also be used to generate the pbias signal. For example, abuffer 510 shown in FIG. 5 may be coupled to the circuitry of FIG. 6 andused to generate the pbias signal.

Accordingly, as shown in FIG. 6, a reference voltage may be generated bya resistor divider. A bias voltage may then be generated using avariable resistor. Although illustrated as a resistor divider andvariable resistor in FIG. 6, generally any electronic components havinga resistance may be used.

As described above, the control voltage used to generate the nbias andpbias signals may vary based on a reference and a frequency of operationof the delay line. The frequency of operation may be obtained in avariety of ways. In some embodiments, intrinsic DLL loop delay may beused to obtain the frequency of operation of the DLL. In some DLLs,circuitry may already be provided for measuring the intrinsic DLL loopdelay, which is related to the period of the signal being propagatedthrough the DLL. FIG. 7 is a schematic illustration of the DLL 200 ofFIG. 2 including an example implementation of the frequency measurementcircuitry 210. A counter 750 may be count a number of clock cycles foran input signal, such as Sig_buff, to propagate through the analog delayline 205. For example, the counter 750 may begin counting clock cyclesresponsive to receiving the Sig_buff signal and may stop responsive toreceiving a corresponding Sig_out output from the analog delay line 205.The count may be indicative of the period of the signal moving throughthe path. That is, a lower count may indicate a slower signal period.The counter may output a digital code corresponding to the count, whichdigital code may be used to adjust the bias signal provided by the biascontrol circuitry. For example, the digital code may be used to set thecurrent generated by the current source 513 of FIG. 5 or the resistanceof the variable resistor 662 of FIG. 6. In some embodiments, the counter750 may already be provided in the DLL 200 to control other aspects ofDLL 200 operation. Accordingly, an existing DLL counter may be used, andan additional counter may not be necessary to implement embodiments ofthe present invention.

The frequency measurement circuitry 200 may have other implementations.For example, a half-cycle analog DLL may be used to lock a half-clockperiod. This may require a charge pump and phase detector be added tothe DLL of FIG. 2. FIG. 8 is a schematic illustration of a DLLincorporating an analog delay line according to an embodiment of thepresent invention. The DLL 800 includes the analog delay line 205 andbuffer 202, as described above. The DLL 800 may further include a phasemixer 802 coupled to the analog delay line 205 for mixing the even andodd output signals to generate a final output signal. As has beendescribed above, digital phase detectors and control logic 207, 209 maybe provided to adjust the delay provided by the analog delay line 205 tosynchronize the input and output signals.

The DLL 800 includes a charge pump phase detector and filter 810configured to generate a signal, V_(freq), indicative of the frequencyof operation of the analog delay line. For example, the charge pumpphase detector and filter 810 may be configured to lock a half-clockperiod. The charge pump phase detector and filter 810 is configured toreceive the buffered input signal from the buffer 202 and anintermediate signal output from one of the analog delay elements in theanalog delay line 205. As shown in FIG. 8, the analog delay line 205might include thirty-two delay elements. The particular illustratedcharge-pump phase detector and filter is configured to receive an outputof the eighth delay element and the buffered input signal. Thecharge-pump phase detector and filter may then generate a signalindicative of frequency V_(freq) based on a comparison of the phases ofthe buffered input signal and the output of the eighth delay element ofthe analog delay line 205. The output of substantially any other elementmay also be used to generate the control voltage. Accordingly, theV_(freq) signal may be indicative of the period of the clock signal, andaccordingly the frequency of operation of the analog delay line. TheV_(freq) signal may be provided to a comparator 815 and compared with afeedback signal V_(fb). The output signal indicative of the comparisonmay be provided to a filter 817 and counter 819. The counter 819 may beconfigured to generate a digital code indicative of the frequency ofoperation of the analog delay line 205. The digital code may be providedto a DAC 821 configured to generate the feedback signal V_(fb) forcomparison with the V_(freq) signal. The DAC may be implemented using,for example, adjustable current mirrors or resistor dividers. Thedigital code may be provided to the bias control circuitry 500 and maybe used to generate the bias control signal. For example, the digitalcode may be used to set the current generated by the current source 513of FIG. 5 or the resistance of the variable resistor 662 of FIG. 6.

FIG. 9 is a schematic illustration of a portion of a memory 900according to an embodiment of the present invention. The memory 900includes an array 902 of memory cells, which may be, for example, DRAMmemory cells, SRAM memory cells, flash memory cells, or some other typeof memory cells. The memory system 900 includes a command decoder 906that receives memory commands through a command bus 908 and generatescorresponding control signals within the memory system 900 to carry outvarious memory operations. The command decoder 906 responds to memorycommands applied to the command bus 908 to perform various operations onthe memory array 902. For example, the command decoder 906 is used togenerate internal control signals to read data from and write data tothe memory array 902. Row and column address signals are applied to thememory system 900 through an address bus 920 and provided to an addresslatch 910. The address latch then outputs a separate column address anda separate row address.

The row and column addresses are provided by the address latch 910 to arow address decoder 922 and a column address decoder 928, respectively.The column address decoder 928 selects bit lines extending through thearray 902 corresponding to respective column addresses. The row addressdecoder 922 is connected to word line driver 924 that activatesrespective rows of memory cells in the array 902 corresponding toreceived row addresses. The selected data line (e.g., a bit line or bitlines) corresponding to a received column address are coupled to aread/write circuitry 930 to provide read data to a data output buffer934 via an input-output data bus 940. Write data are applied to thememory array 802 through a data input buffer 944 and the memory arrayread/write circuitry 930.

A clock signal generator 950 is configured to receive an external clocksignal and generate a synchronized internal clock signal in accordancewith embodiments of the present invention. The clock signal generator950 may include, for example, the DLL 200 if FIG. 2. The clock signalgenerator 950 may receive an external clock signal applied to the memorysystem 900 and may generate a synchronized internal clock signal whichmay be supplied to the command decoder 906, address latch 910, and/orinput buffer 944 to facilitate the latching of command, address, anddata signals in accordance with the external clock.

Memory systems in accordance with embodiments of the present inventionmay be used in any of a variety of electronic devices including, but notlimited to, computing systems, electronic storage systems, cameras,phones, wireless devices, displays, chip sets, set top boxes, or gamingsystems.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention.

What is claimed is:
 1. An analog delay line system comprising: an analogdelay line comprising a plurality of analog delay elements, individualones of the plurality of analog delay elements configured to receive abias signal and a respective periodic input signal and delay therespective periodic input signal to provide a periodic output signalgenerate a respective delayed periodic output signal, wherein therespective periodic input signal of each one of the delay elementscoupled to an output of another one of the delay elements comprises arespective one of the delayed periodic output signals, and wherein anamount of delay is based, at least in part, on the bias current signal;frequency measurement circuitry coupled to at least a portion of theanalog delay line and configured to measure a frequency of at least oneof said respective periodic input signals, the frequency measurementcircuitry configured to provide a control signal based, at least inpart, on the frequency of the at least one of said respective periodicinput signals; and bias control circuitry configured to receive thecontrol signal and further configured to generate the bias signal based,at least in part, on the frequency of the at least one of saidrespective periodic input signals.
 2. The analog delay line system ofclaim 1, wherein individual ones of the plurality of analog delayelements comprise a respective delay amplifier.
 3. The analog delay linesystem of claim 1, wherein individual ones of the plurality of analogdelay elements comprise no more than one respective delay amplifier. 4.The analog delay line system of claim 1, wherein individual ones of theanalog delay elements further include a multiplexer configured to selectan output of the respective analog delay element as an output of theanalog delay line in accordance with a multiplexer control signal. 5.The analog delay line system of claim 4, wherein multiple ones of themultiplexers are coupled to a same load circuit.
 6. The analog delayline system of claim 4, wherein multiple ones of the multiplexers arecoupled to a same bias circuit.
 7. The analog delay line system of claim1, wherein the analog delay line is further configured to generate evenand odd output periodic signals, and wherein the analog delay linesystem further comprises: an analog phase mixer configured to mix theeven and odd output periodic signals.
 8. The analog delay line system ofclaim 1, further comprising: digital control logic configured togenerate at least one digital control signal and wherein at leas tone ofthe analog delay elements is configured to receive the at least onedigital control signal and wherein the amount of delay provided by theanalog delay element is also based, at least in part, on the digitalcontrol signal.
 9. The analog delay line system of claim 1, wherein thefrequency measurement circuitry comprises: a counter configured togenerate a signal corresponding to a number of periods elapsed betweenan input and an output of the analog delay line; and wherein the biascontrol circuitry is configured to generate the bias based, at least inpart, on the signal corresponding to the number of periods elapsedbetween an input and an output of the analog delay line.
 10. The analogdelay line system of claim 1, wherein the frequency measurementcircuitry comprises: a charge pump phase detector configured to generatethe control signal indicative of a phase difference between at least onerespective periodic input signal and at last one respective periodicoutput signal; and wherein the bias control circuitry is configured togenerate the bias, at last in part, on the control signal.
 11. Theanalog delay line system of claim 1, wherein the bias control circuitrycomprises: a bandgap reference; and an adjustable current source,wherein the adjustable current course is configured to generate acurrent based, at least in part, on a comparison of the frequency withthe bandgap reference, wherein the bias is generated based, at least inpart, on the current.
 12. A system comprising: an analog delay lineconfigured to receive a bias and an input signal, and delay the inputsignal to provide a delayed output signal based, at least in part, onthe bias; frequency measurement circuitry configured to measure afrequency of operation of the analog delay line and to provide anindication of the frequency of operation; and bias control circuitryconfigured to receive the indication of the frequency of operation andto generate the bias, wherein the bias is based, at least in part, onthe frequency of operation.
 13. The system of claim 12, wherein thefrequency of operation comprises the frequency of the input signal. 14.The system of claim 12, wherein the bias comprises a bias voltage. 15.The system of claim 12, wherein the bias comprises a bias current. 16.The system of claim 12, wherein the analog delay line comprises aplurality of delay elements and further comprises a bias circuit,wherein the bias circuit is configured to provide a bias current to morethan one of the delay elements based, at least in part, on the bias. 17.An adaptive bias circuit for use with an analog delay line, the adaptivebias circuit comprising: an operational amplifier configured to receivea control signal and a feedback signal and further configured togenerate a first bias signal based, at least in part on a comparison ofthe control signal and the feedback signal; a first transistorconfigured to receive the control signal at a gate of the firsttransistor; and a second transistor having a gate coupled to asource/drain terminal of the second transistor, and wherein thesource/drain of the second transistor is coupled to source/drain of thefirst transistor, and wherein the second transistor is configured togenerate the feedback signal at the gate of the second transistor,wherein the gate of the second transistor is coupled to the operationalamplifier to provide the feedback signal to the operational amplifier.18. The adaptive bias circuit of claim 17, further comprising a bufferconfigured to receive the first bias signal, wherein the bufferincludes: a third transistor configured to receive the first bias signalat a gate of the third transistor; and a fourth and a fifth transistor,wherein a source/drain of the fourth transistor is coupled to asource/drain of the fifth transistor and to a source/drain of the thirdtransistor, and wherein a gate of the fourth transistor and a gate ofthe fifth transistor are coupled to the source/drain of the fifthtransistor, and wherein the gate of the fifth transistor is configuredto generate a second bias signal.
 19. The adaptive bias circuit of claim17, further comprising a control generator coupled to the operationalamplifier and configured to generate the control signal, wherein thecontrol generator comprises: a variable current source; a thirdtransistor having a gate coupled to the variable current source and afirst source/drain of the third transistor coupled to a supply voltage;and a fourth transistor having a gate coupled to the supply voltage, afirst source/drain of the fourth transistor coupled to the variablecurrent source and a second source/drain of the fourth transistorcoupled to a second source/drain of the third transistor, and whereinthe second source/drain of the fourth transistor is configured togenerate the control signal.
 20. The adaptive bias circuit of claim 19,wherein the variable current source provides a current based, at leastin part, on a frequency of operation of the analog delay line.
 21. Theadaptive bias circuit of claim 20, further comprising: a bandgapreference coupled to the variable current source, and wherein thevariable current source further provides the current based, at least inpart, on the bandgap reference.
 22. The adaptive bias circuit of claim21, further comprising: a digital to analog converter coupled to thevariable current source, and configured to receive a signal indicativeof the frequency of operation of the analog delay line, the digital toanalog converter configured to output a signal indicative of a currentadjustment; and wherein the variable current source is furtherconfigured to receive the signal indicative of a current adjustment andfurther provide the current based, at least in part, on the signalindicative of the current adjustment.
 23. The adaptive bias circuit ofclaim 20, further comprising a counter coupled to the variable currentsource and configured to couple to the analog delay line generate asignal corresponding to a number of periods elapsed between an input andan output of the analog delay line; and wherein the variable currentsource is further configured to provide the current based, at least inpart, on the signal corresponding to the number of periods elapsedbetween an input and an output of the analog delay line.
 24. A method ofbiasing an analog delay line, the method comprising: measuring afrequency of operation of the analog delay line; generating a biasbased, at least in part, on the frequency of operation of the analogdelay line; and providing the bias to the analog delay line, wherein theanalog delay line is configured to delay an input signal by an amountbased, at least in part, on the bias.
 25. The method of claim 24,wherein the act of generating the bias comprises adaptively generatingthe bias based, at least in part, on the frequency of operation of theanalog delay line.
 26. The method of claim 24, wherein the act ofmeasuring the frequency of operation of the analog delay line comprisesmeasuring a frequency of an input signal to the analog delay line. 27.The method of claim 24, wherein the act of measuring the frequencyoccurs in a start-up mode of operation of the analog delay line.
 28. Themethod of claim 24, wherein the act of generating a bias furthercomprises generating a control voltage based, at least in part, on abandgap reference and an output of a digital-to-analog converter. 29.The method of claim 24, wherein the act of measuring a frequency ofoperation comprises counting a number of periodic signal cycles from aninput to an output of the analog delay line.
 30. The method of claim 24,wherein the act of measuring a frequency of operation comprisescomparing a phase of an input periodic signal and an intermediateperiodic signal of the analog delay line.
 31. The method of claim 24,wherein the analog delay line comprises a plurality of analog delayelements coupled in series, individual ones of the analog delay elementsincluding an analog amplifier and a multiplexer.
 32. The method of claim31, wherein the act of providing the bias comprises providing the biasto bias circuitry shared by individual ones of the analog amplifiers.